Those skilled in the arts of antenna arrays and beamformers know that antennas are transducers which transduce electromagnetic energy between unguided- and guided-wave forms. More particularly, the unguided form of electromagnetic energy is that propagating in “free space,” while guided electromagnetic energy follows a defined path established by a “transmission line” of some sort. Transmission lines include coaxial cables, rectangular and circular conductive waveguides, dielectric paths, and the like. Antennas are totally reciprocal devices, which have the same beam characteristics in both transmission and reception modes. For historic reasons, the guided-wave port of an antenna is termed a “feed” port, regardless of whether the antenna operates in transmission or reception. The beam characteristics of an antenna are established, in part, by the size of the radiating portions of the antenna relative to the wavelength. Small antennas make for broad or nondirective beams, and large antennas make for small, narrow or directive beams. When more directivity (narrower beamwidth) is desired than can be achieved from a single antenna, several antenna elements may be grouped together into an “array” and fed together in a phase-controlled manner, to generate the beam characteristics of an antenna larger than that of any single antenna element, and which is therefore capable of generating a narrower beam than a single antenna element of the array. The structures which control the apportionment of power to (or from) the antenna elements are termed “beamformers.” A beamformer includes a beam port and a plurality of element ports. In a transmit mode, the signal to be transmitted is applied to the beam port and is distributed by the beamformer to the various element ports. In the receive mode, the unguided electromagnetic signals received by the antenna elements and coupled in guided form to the element ports are combined to produce a beam signal at the beam port of the beamformer. A salient advantage of sophisticated beamformers is that they may include a plurality of beam ports, each of which distributes the electromagnetic energy in such a fashion that different beams may be generated simultaneously.
A large number of methods and apparatus have been used to generate electronically scanned antenna beams using phased arrays made up of, or comprising, multiple radiating elements. A phased array can be formed as a linear array, as illustrated in FIG. 1, in which the radiating elements of the array are in-line. In FIG. 1, a linear array 10 includes a set 12 of antennas 120, 12φ, 122φ, 123φ, 124φ, 125φ, 126φ, 127φ, 128φ, and 129φ. Each of the antennas of set 12 of antennas is coupled by way of a phase shifter of a set 14 of phase shifters to a source of signal of a set 16 of sources. More particularly, antenna 120 is coupled by way of a phase shifter 140 to a source 160. Antenna 12φ is coupled by way of a phase shifter 14φ to a signal source 16φ, antenna 122φ is coupled by way of a phase shifter 142φ to a signal source 162φ, antenna 123φ is coupled by way of a phase shifter 143φ to a signal source 163φ, antenna 124φ is coupled by way of a phase shifter 144φ to a signal source 164φ, antenna 125φ is coupled by way of a phase shifter 145φ to a signal source 165φ, antenna 126φ is coupled by way of a phase shifter 146φ to a signal source 166φ, antenna 127φ is coupled by way of a phase shifter 147φ to a signal source 167φ, antenna 128φ is coupled by way of a phase shifter 148φ to a signal source 168φ, and antenna 129φ is coupled by way of a phase shifter 149φ to a signal source 169φ. Each source of set 16 of signal sources of FIG. 1 is at the same frequency f0 and in-phase. Each phase shifter of set 14 of signal sources of FIG. 1 is set to create the progressive phase shift. More particularly, phase shifter 140 provides some phase shift as a reference to the other phase shifters. Phase shifter 14φ provides phase shift equal to the reference phase plus φ as shown in FIG. 1. Phase shifter 142φ provides phase shift equal to the reference plus 2φ and so on. So the array beam peak is steered to the angle
                              sin          ⁢                                          ⁢                      θ            0                          =                              φ            ⁢                                                  ⁢                          λ              0                                            2            ⁢                                                  ⁢            π            ⁢                                                  ⁢            D                                              (        1        )            where:                D is the physical separation between antenna elements, or more properly between their phase centers;        λ0 is the wavelength at the carrier frequency f0; and        θ0 is the angle to which the beam peak is steered or guided.        
A phased array can also be formed as a planar array, such as that illustrated as 20 of FIG. 2. In FIG. 2, planar array 20 includes six rows I, II, III, IV, V, and VI of antenna arrays, any one of which may be similar to array 10 of FIG. 1. In FIG. 2, row I is illustrated as including the antennas of array 10 of FIG. 1. Of course, planar array 20 may contain more or fewer rows than six, and each row may contain more or fewer antennas than nine.
Linear phased arrays may also be subdivided into overlapping and nonoverlapping subarrays. A nonoverlapping array is illustrated by 30 in FIG. 3. In FIG. 3, antenna array 30 includes a linear array of 32 antenna elements included within eight antenna subarrays of a set 31 of subarrays. Of course, array 30 may contain more or fewer than 4 elements in subarray and more than 8 subarrays. Each row 30 can be part of planar array similar to FIG. 2. Each subarray of set 31 is fed by a summing or dividing circuit (E) of a set 38 of subarrays. Each summing circuit of set 38 of summing or dividing circuits is, in turn, fed by a phase shifter of a set 34 of phase shifters from a source of a set of sources 36. More particularly, a subset 311 of antenna element subset 31 includes antenna elements 321, 322, 323, and 324, each of which is fed from an output port of a summing or dividing circuit 381. The common port 381c of summing or dividing circuit 381 is fed by a phase shifter 341 from a source 361. Similarly, a subset 312 of antenna element subset 31 includes antenna elements 325, 326, 327, and 328, each of which is fed from an output port of a summing or dividing circuit 382. The common port 382c of summing or dividing circuit 382 is fed by a phase shifter 342 from a source 362. A subset 313 of antenna element subset 31 includes antenna elements 329, 3216, 3211, and 3212, each of which is fed from an output port of a summing or dividing circuit 383. The common port 383C of summing or dividing circuit 383 is fed by a phase shifter 343 from a source 363. There may be many subarrays in the array 30 of FIG. 3, and the last subarray is illustrated as 31N. Subarray 31N includes antenna elements 3229, 3230, 3231, and 3232, each of which is fed from an output port of a summing or dividing circuit 38N. The common port 38NC of summing or dividing circuit 38N is fed by a phase shifter 34N from a source 36N. As in the case of the array of FIG. 1, all the sources of set 36 of sources are at the same frequency and phase.
Linear phased arrays may also be combined into overlapping subarrays, as illustrated in simplified form by array 40 in FIG. 4. In FIG. 4, overlapping linear array 40 includes 32 antenna elements, illustrated on different lines to clarify the subdivisions. The uppermost line 411 includes twelve mutually adjacent elements. The left-most 4 elements in line 411 are fed at a frequency of f0 by a phase shifter and source 421. The next 4 elements in line 411 (elements 5 through 8 from the left) are fed simultaneously from first phase shifter and source 421 and a second phase shifter and source 422. The last 4 elements in line 411 (elements 9 through 12 from the left) are fed simultaneously from first phase shifter and source 421, second phase shifter and source 422 and a third phase shifter and source 423. In the second-from-top line 412 of antenna elements, four additional antenna elements (at the right of the line) are fed simultaneously from phase shifter and sources 422, 423, and 424. The third-from-top line 413 of antenna elements includes four additional antenna elements, all of which are fed simultaneously from phase shifter and sources 423, 424, and 425. The fourth-from-top line 414 of antenna elements includes four additional antenna elements, all of which are fed simultaneously from phase shifter and sources 424, 425, and 426. The fifth-from-top line 415 of antenna elements includes four additional antenna elements, all of which are fed simultaneously from phase shifter and sources 425 and 426. The right-most four antenna elements of row 416 are fed from phase shifter and source 426.
Another known approach to electronic scanning is a technique which may be referred to as ultra-fast “within-pulse-electronic-sector-scanning” (WPESS), which can be found in various references, such as
[1]U.S. Pat. No. 2,426,460August 1947 Lewis[2]U.S. Pat. No. 3,012,244November 1961Langenwalter et al.[3]U.S. Pat. No. 5,943,010August 1999Rudish et el.[4]U.S. Pat. No. 5,541,607July 1996 Reinhardt[5]U.S. Pat. No. 6,531,976March 2003 Yu[6]U.S. Pat. No. 6,624,783September 2003 Rabideau[7]U.S. Pat. No. 6,778,137August 2004Krikorian et al.
Samuel M. Sherman, Monopulse Principles and Techniques, Artech House, Norwood, Mass., 1984
Within-pulse-electronic-sector-scanning (WPESS) is based on progressive frequency offset between the adjacent radiating elements. Progressive frequency offset is illustrated in FIG. 5 for the case of a linear array 512 of antenna elements. In FIG. 5, linear array 512 includes antenna elements designated 5120, 5121, 5122, 5122, 5124, 5125, 5126, 5127, 5128, and 5129. Each antenna element of FIG. 5 is driven by a signal source at a frequency defined by the frequency shown on the bottom of each antenna. For example, antenna element 5120 is driven at a frequency f0, antenna element 5121 is driven at a frequency f0−Δf, antenna element 5122 is driven at a frequency f0−2Δf, antenna element 5123 is driven at a frequency f0−3Δf, antenna element 5124 is driven at a frequency f0−4Δf, antenna element 5125 is driven at a frequency f0−5Δf, antenna element 5126 is driven at a frequency f0−6Δf, antenna element 5127 is driven at a frequency f0−7Δf, antenna element 5128 is driven at a frequency f0−8Δf, and antenna element 5129 is driven at a frequency f0−9Δf. The differential phase shift between far field components radiated by antenna 5120 and antenna 512n isφn=−2πnΔft+nkd sin θ  (2)and includes time-dependent progressive phase shift with decrement −2πΔft. So the linear array 512 beam peak position becomes determinable from
                              sin          ⁢                                          ⁢                                    θ              0                        ⁡                          (              t              )                                      =                                            2              ⁢                                                          ⁢              π              ⁢                                                          ⁢              Δ              ⁢                                                          ⁢              f                                      k              ⁢                                                          ⁢              d                                ⁢          t                                    (        3        )            Therefore, all the phase shifters of set 14 in FIG. 1 can be removed and replaced by the set of local oscillators with frequency offset Δf, 2Δf, 3Δf . . . as shown in FIG. 5. The beam steering velocity and angular coverage is controlled by the value of frequency offset and must be matched with pulse duration. The f0 carrier oscillator driving antenna 5120 of FIG. 5 may be viewed as being a phase or frequency reference to which other phases or frequencies may be referred. The phase or frequency reference may be at a location within the array, rather than at an end thereof as illustrated.
A full scan of a sector of several tens of degrees is usually required for any volume search radar (VSR). If this sector is covered during a single pulse using Within-pulse-electronic-sector-scanning (WPESS) technique, all targets within this angular sector are illuminated in sequence and the return signals therefrom can be processed. The main disadvantage of the “within-pulse-electronic-sector-scanning” (WPESS) architecture of the prior art is poor signal-to-noise ratio. This poor signal-to-noise ratio occurs because fast scan means short time of target illumination, and consequently results in less energy being reflected toward the receiver from the target. Some signal-to-noise ratio loss can be reduced by integration of several returns from the target. However, this multi-pulse processing is limited in accuracy by the scintillation effect, glint, propagation and multipath errors, jamming, and the like, as set forth in reference [8].
Improved or alternative electromagnetic target acquisition and tracking techniques are desired.